Add support for NextToLast shard function.

flatfs.go

@@ -62,6 +62,15 @@ func Suffix(suffixLen int) ShardFunc {
 	}
 }
 
+func NextToLast(suffixLen int) ShardFunc {
+	padding := strings.Repeat("_", suffixLen+1)
+	return func(noslash string) string {
+		str := padding + noslash
+		offset := len(str) - suffixLen - 1
+		return str[offset : offset+suffixLen]
+	}
+}
+
 func (fs *Datastore) encode(key datastore.Key) (dir, file string) {
 	noslash := key.String()[1:]
 	dir = path.Join(fs.path, fs.getDir(noslash))

flatfs_test.go

@@ -33,6 +33,7 @@ func tempdir(t testing.TB) (path string, cleanup func()) {
 func tryAllShardFuncs(t *testing.T, testFunc func(mkShardFunc, *testing.T)) {
 	t.Run("prefix", func(t *testing.T) { testFunc(flatfs.Prefix, t) })
 	t.Run("suffix", func(t *testing.T) { testFunc(flatfs.Suffix, t) })
+	t.Run("next-to-last", func(t *testing.T) { testFunc(flatfs.NextToLast, t) })
 }
 
 func TestPutBadValueType(t *testing.T) {
@@ -230,6 +231,14 @@ func TestStorage(t *testing.T) {
 			dirFunc: flatfs.Suffix,
 		}, t)
 	})
+	t.Run("next-to-last", func(t *testing.T) {
+		testStorage(&params{
+			what:    "next-to-last",
+			dir:     "uu",
+			key:     "quux",
+			dirFunc: flatfs.NextToLast,
+		}, t)
+	})
 }
 
 func testHasNotFound(dirFunc mkShardFunc, t *testing.T) {